1. Field of the Invention
The present invention relates to light emitting diodes (LEDs) and more particularly to LEDs having improved current spreading structures.
2. Description of the Related Art
LEDs are an important class of solid state devices that convert electric current to light. They generally comprise an active layer of semiconductor material sandwiched between two oppositely doped layers, one being p-type and the other being n-type. A drive current is applied across electrical contacts on the doped layers causing electrons and holes to be injected from the doped layers into the active layer. The electrons and holes then recombine to generate light that emits omnidirectionlly from the active layer and escapes from all surfaces of the LED.
One disadvantage of most conventional LEDs is that they are less efficient at converting current to light than are filament lights. As a result, their use has most often been limited to applications such as indicating lamps in electronic devices, where the LED's die size is less than 0.25 mm and where optical power is less than 10 milliwatts (mW).
However, recent advances in nitride based semiconductor materials has led to the development of bright, highly efficient LEDs emitting in the blue-green spectral region which can be used to generate various colors of light, including white light. [See Nichia Corp. white LED, Part No. NSPW300BS, NSPW312BS, etc.; See also U.S. Pat. No. 5,959,316 to Hayden, “Multiple Encapsulation of Phosphor-LED Devices”]. These advancements have led to solid state emitters for use in lighting and signaling applications that require high output power and high luminous flux. One such application is traffic signals. Current LED traffic signals consist of an array of single LED devices combined to obtain high output power. However, a single high-power LED device that can replace the LED array would be less complex, cost less and would be more reliable.
One way to increase the power and luminous flux of an LED is to increase its size and emitting surface area. However, the size of conventional nitride based LEDs is limited by the inability of current to effectively spread from the electrical contacts to the active layer. P-type nitride based semiconductor materials have relatively poor conductivity, and current applied to the p-type contact will only spread to a limited area within the p-type layer. The current will not migrate to the entire active layer, and the LED can experience local heating and premature degradation around the contact.
N-type nitride based semiconductor materials are better conductors but still present some resistance to the spread of current. As the device size increases, the material's ability to uniformly spread current from the n-type contact is reduced. As a result, the size of nitride base LEDs is limited by the both the p- and n-type layers' current spreading characteristics.
Various LEDs have been developed with structures to increase current spreading [See G. B. Stringfellow and M. G. Crawford (1997), High Brightness Light Emitting Diodes, Semiconductors and Semimetals, Vol. 48, Pages 170-178]. The devices generally include an n-type epitaxial layer grown on a conductive substrate, with a LED active region and p-type layer grown on the n-type layer. A conductive contact is deposited on the center of the p-type layer's surface and a conductive contact pad is deposited on the conductive substrate opposite the epitaxial layer. Current from the p-type contact spreads from the center towards the edges of the p-type layer, and then to the active layer. The substrate is very thick compared to the epitaxial layers and as a result, the overall current spreading into the active region is limited by the spreading provided by the p-type contact. This basic structure is effective for small LEDs (approximately 0.25 mm2), but is not scalable to larger LEDs. In order to facilitate LED size scaling, modifications to the LED must be made.
One such modification increases the thickness of the p-type layer to decrease its spreading resistance so that current spreads to the edge of the LED. This approach is effective in increasing the LED area, but the LED scaling is practically limited because the p-type layer thickness cannot be increased indefinitely. Also, for the GaN-based LED system, the p-type material has very low conductivity, making this approach impractical.
In another approach, contacts have been deposited in the center of the p-type layer's surface with thin radial conductive fingers running from the contact toward the edge of the surface. Current applied to the contact spreads to the conductive fingers and to the p-type surface below. While an improvement, the LED still cannot be freely scaled to large sizes. As the size increases, the distance between the ends of the radial fingers increases and a point is reached at which this distance prevents current from spreading throughout the p-type layer. This structure also cannot be used on LEDs fabricated on an insulating substrate.
U.S. Pat. No. 5,652,434 to Nakamura et al. discloses a structure that improves current spreading in nitride based LEDs grown on insulating substrates. It comprises an LED structure on the insulating substrate, with the n-type layer adjacent to the substrate and the p-type layer on the epitaxial layer surface. Because the substrate is insulating, a contact pad cannot be used to spread current through the substrate and to the n-type layer. Instead, a corner of the LED structure is etched through the p-type layer, the active layer, and partially into the n-type layer. A contact is deposited on the etched area so that current applied to the contact spreads through the relatively conductive n-type material. To spread current across the p-type layer, a semi-transparent current spreading layer is deposited on the p-type layer. A p-type contact is deposited on the spreading layer in the corner of the LED opposite the n-type contact. The current applied to the p-type contact will spread through the spreading layer and to the p-type layer below it.
This structure provides an improvement in spreading current in standard size devices, but cannot efficiently spread current in larger sized LEDs. Because the p-type layer is an LED surface, the spreading layer should be as thin as possible so that it will not absorb emitted light. However, the thinner the spreading layer the greater its sheet resistance. As the LED size increases the sheet resistance prevents the current from fully spreading across the p-type layer. The spreading layer's sheet resistance can be reduced by using semi-transparent metallic materials, and/or increasing its thickness. However, these changes would reduce transparency and increase light absorption, reducing the LED's light output.
Also, the increased spreading resistance in the n-type layer can cause excessive heating, and prevent full current spreading and uniform light output. To reduce the spreading resistance, the thickness of the n-type layer can be increased as the device size increases. However, this significantly increases the necessary materials and process times, both of which can result in prohibitive cost increases.